1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device including a semiconductor layer made of gallium nitride (GaN).
2. Description of the Related Art
Nitride semiconductor such as gallium nitride (GaN) is applied in practice to a semiconductor light emitting device such as a light emitting diode (LED) and a semiconductor device such as a high-speed transistor capable of operating at high temperature.
At the time of manufacturing such a semiconductor device using the nitride semiconductor, a nitride semiconductor layer is grown to a predetermined thickness on a base-material substrate such as a sapphire substrate and, after that, the interface between the nitride semiconductor layer and the base-material substrate is peeled off, thereby obtaining a nitride semiconductor substrate. For example, the nitride semiconductor layer and the base-material substrate are separated by applying a laser beam having predetermined strength to the interface between the nitride semiconductor layer and the base-material substrate to locally heat and sublimate the laser beam applied part. By using the sublimation effect of such laser beam application, a semiconductor device is efficiently manufactured.
At the time of separation between the nitride semiconductor layer and the base-material substrate by the laser beam, during the process, only the part irradiated with the laser beam therebetween is separated, and the other part of them remains joined. Consequently, the possibility of occurrence of stress concentration in the joined part therebetween and occurrence of a crack in the surface of the nitride semiconductor layer is high.
A surface layer in which a crack occurs is called a damaged layer. The existence of the damaged layer causes deterioration in the physical strength and characteristics and may also cause contamination and adhesion of impurity during manufacturing process. In the case of manufacturing a semiconductor light emitting device, an electrode has to be attached to a nitride semiconductor layer. In the case where the surface layer of the nitride semiconductor layer is a damaged layer, contact resistance of the electrode tends to increase. Further, in the case of dry-etching the damaged layer on the surface in order to process the electrode, a shortcoming occurs such that a micro pillar structure called a pillar is generated.
To address the shortcoming, the applicant of the present invention developed a technique of removing a damaged layer by performing CMP (Chemical Mechanical Processing) after growing the nitride semiconductor layer as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2006-86388.